Ferrous alloys

ABSTRACT

A HEAT TREATED CAST FERROUS ALLOY CHARACTERIZED BY HIGH CHROMIUM AND CARBON CONTENT AND HAVING A METALLOGRAPHIS STRUCTURE EVIDENCING A HIGH CONTENT OF HARD EUTECTIC AND HYPEREUTECTIC CHROMIUM CARBIDES AND A MARTENSITIC SOLID SOLUTION FREE OF SECONDARY CARBIDES. THE METALLOGRAPHIC STRUCTURE IS OBTAINED BY THE SELECTION OF ALLOYING ELEMENTS IN THE PROPER PROPORTION AND HEAT TREATMENT.

M. J. FELTZ FERROUS ALLOYS Sept. 10, 1974 Filed June 2'7, 1972 United States Patent 3,834,950 FERROUS ALLOYS Michel Joseph Feltz, 14c Rue Hotteux, Ayeneux, Belgium Filed June 27, 1972, Ser. No. 266,703 Claims priority, application Luxembourg, June 29, 1971,

Int. Cl. C22c 39/14, 39/20 U.s.c1. 148-31 8 Claims ABSTRACT OF THE DISCLOSURE A heat treated cast ferrous alloy characterized by high chromium and carbon content and having a metallographic structure evidencing a high content of hard eutectic and hypereutectic chromium carbides and a martensitic solid solution free of secondary carbides. The metallographic structure is obtained by the selection of alloying elements in the proper proportion and heat treatment.

BACKGROUND OF THE INVENTION (1) Field of the Invention (2) Description of the Prior Art High chromium-carbon ferrous alloys have long been known to be particularly useful for the manufacture of elements and equipment parts which are to be subjected to abrasion, corrosion, hot oxidation and repeated impacts. Thus, for example, high chromium-carbon alloys are particularly well suited for use as grinding balls, cylpebs, lining plates, screens, classification parts, grate bars for furnaces, parts of turbines, etc.

While not limited thereto in its utility, the present invention will be discussed below in relation to the dry or wet grinding of material. Elements employed in grinding mills, such as lining plates and more particularly grinding balls, are exposed to exceedingly severe conditions. A large number of materials; including various ores, coal, coke, sand, phosphates, etc.; are presently ground in ball mills. In the operation of such ball mills, in accordance with the prior art, balls manufactured from iron and castiron alloys are employed as the grinding media. These prior art grinding balls are consumed at a comparatively high rate, for example, in the range of a few grams to several kilos per ground ton of material. It has long been desired to reduce this comparatively high grinding media consumption rate.

Numerous studies have shown that the wear resistance of a ferrous alloy may be improved by use of a high carbon alloy containing as many carbides as possible. However, the carbide content of the alloys employed in the prior art for the manufacture of grinding media has been limited by the existence of a eutectic phase in the Fe-C or Fe-C-Cr equilibrium diagram. Thus, low alloyed white cast irons containing, by way of example, 0-10% chromium are limited to a Fe C carbide content slightly in excess of 50%. If, during the casting of such a hypereutectic alloy, the carbide content is increased significantly above 50%, the resulting grinding media obtained are extremely brittle and cannot be employed since the mechanical properties and resistance of such alloys are insuflicient to withstand repeated impacts.

Ferrous alloys containing a higher content of chromium, that is with 1030% chromium, have been gaining in acceptance due to the enhanced hardness of such alloys in the martensitic state. These higher chromium alloyed cast irons, however, have a useful limit of about 35% of chromium carbides of the (Fe, Cr) C type. Thus, while the higher chromium content alloyed cast irons have carbides which are harder than the cementite (Fe C) of the alloys characterized by less than 10% chromium, the maximum percentage of carbides which can be employed without rendering the alloys too brittle is reduced. As in the case of the alloys with the lower chromium content, trying to increase the carbide content by casting a hypereutectic alloy will result in brittleness and thus these 10-30% chromium alloys cannot be successfully employed for grinding balls and other grinding media.

SUMMARY OF THE INVENTION The present invention overcomes the above briefly discussed and other disadvantages and deficiencies of the prior art by providing novel and improved ferrous alloys with a high content of chromium and carbon and characterized by a high content of hard primary eutectic and hypereutectic chromium carbides in a martensitic matrix. The alloys in accordance with the invention, accordingly, have significantly improved wear resistance properties when compared to the prior art while simultaneously having mechanical properties and impact resistance sufiiciently high to resist the repeated impacts encountered by grinding media in, for example, ball mills.

The ferrous alloys of the present invention have a composition by weight corresponding to the following analysls:

Carbon From 2.3 to 5%. Chromium 33 to 51%. Manganese 0.15 to 1.5%. Silicon 0.15 to 1.5%.

the balance being substantially iron with the usual impurity contents for commercial cast iron, mainly sulphur and phosphorous. As employed herein, the phrase usual impurity contents is intended to encompass sulphur within the range of 0.01 to 0.15% and phosphorous within the range of 0.01 to 0.15%. The carbon and chromium contents of the alloys in accordance with the present invention are bound by the following relationship:

Percent Cr8 percent 0:11 to 16 The ferrous alloys of the invention are also characterized by a metallographic structure constituted by a martensitic solid solution without ferrite and containing eutectic and hypereutectic carbides, but being free of secondary proeutectoid carbides; such alloys having a hardness equal or superior to 60 on the Rockwell C scale.

Alloys in accordance with the present invention, having a composition within the bounds set forth above, contain chromium carbides which are mainly of the (Fe, Cr) C type. Such chromium carbides are of relatively small dimensions and thus form a close bond with the matrix.

In accordance with the present invention chromium alloyed cast irons, having contents of chromium carbide ranging from 35 to and showing a hypereutectic structure mating perfectly into the matrix, may be cast and the resulting products are particularly well suited for the manufacture of grinding media.

As cast, alloys in accordance with the invention have a mainly ferritic matrix and a hardness which ranges from 30 to 45 on the Rockwell C scale depending on the primary carbide content. Such cast alloys may be easily machined. Further, such alloys, despite their ferritic struc ture, can be submitted to a hardening heat-treatment to convert the ferrite into hard martensite by austenitization.

at temperatures in the range of 1025 to 1150 C. The duration of the austenitization is preferably between 1 and hours depending upon the dimensions of the piece being treated and cooling can be performed in either blown or still air. After hardening the alloys have a hardness equal to or superior to r 60 on the Rockwell C scale and, in some cases, hardness in the range of R 63 to 66 has been measured.

During the austenitization process an exchange of carbon and chromium takes place between the primary carbides and the matrix with carbon moving from the carbides to the matrix and chromium from the matrix to the carbides. During this exchange the structure of the hypereutectic carbides in the alloy changes, at least on the surface, thereby apparently resulting in the excellent bond of the carbides to the matrix which is characteristic of the product.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawing wherein:

FIG. 1 depicts the microstructure of an alloy in accordance with the invention after hardening, FIG. 1 being at a magnification of 200; and

FIG. 2 depicts the alloy of FIG. 1 with a magnification of 500.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, the depicted microstructure shows the hypereutectic and eutectic carbides and the matrix which is a martensitic solid solution containing an amount of residual austenite but no ferrite. It is noteworthy that the alloys in accordance with the invention do not contain secondary pro-eutectoid carbides and thus have improved corrosion resistance, for example during grinding by wet processes, when compared to the prior art. Martensitic cast irons which were free of secondary carbides have not previously been available. Thus, one of the particularly unique features of the present invention is the provision of high chromium and carbon alloys of martensitic metallographic structure containing eutectic and hypereutectic carbides and being free of secondary carbides.

The alloys in accordance with the present invention, after hardening in blown or still air, may be subjected to a stress-relief heat treatment in the range of 100 to 300 C. to eliminate residual stresses. Subsequent to the hardening step, in order to decrease the residual austenite content, it may in some cases be advisable to subject the alloys to a tempering heat-treatment in the range of 450 to 550 C. or to a cold treatment which may reach 200 C. below zero. In the case of a cold treatment, the residual tensions may also be removed by a stress-relief heat-treatment in the 100 to 300 C. range.

A preferred composition of the invention, within the bounds set forth above, is characterized by the following analysis by weight:

Percent C,- 37 Mn 0.5 Si 0.

peeling of the balls after several thousand hours of opcal properties and impact resistance are slightly inferior to those of the above described preferred composition, are characterized by the following analyses by weight:

Percent C 3.5

Cr 41.0 Mn 0.5 Si 0.4

and

Percent C Cr 45.0

the balance of these two additional alloys each being mainly iron with small quantities of impurities such as phosphorous and sulphur.

According to another aspect of the invention the wear resistance of the alloys can be improved by adding, to an alloy selected Within the above stated range of the four principal components, molybdenum in an amount of 0 to 2.5%, niobium in an amount between 0 to 2.5 or both elements simultaneously in a total amount between 0 to 5% wherein neither one of the added alloy elements individually exceeds 2.5% by weight. Thus, the following compositions have proved particularly advantageous:

Percent C 3 Si 0.4 Mo 15 and Percent Mn 0.5 Si 0.4

the percentages being by weight and the balance of both alloys being mainly iron with small amounts of impurities such as phosphorous and sulphur.

The following table lists the results obtained with wear tests performed with grinding balls in quartz sand:

TABLE I Wear resistance Alloy: factor Pearlitic white iron with 3% C Martensitic Ni-Cr white iron Alloy 3% C, 37% Cr, 0.5% Mn, 0.4% Si 250 Alloy 3.5% C, 41% Cr, 0.5% Mn, 0.4% Si 275. Alloy 4% C, 45% Cr, 0.5% Mn, 0.4% Si 300 Alloy 3% C, 36% Cr, 0.5% Mn, 0.4% Si, 1.5%

Mo 280 Alloy 3% C, 35% Cr, 0.5% Mn, 0.4% Si, 1%

Nb 280 1 Reference.

As can be seen from Table I, the alloys according to the invention, with or without molybdenum and niobium, have clearly superior wear resistance when compared to the prior art. The present alloys also have excellent wear resistance when exposed to corrosion phenomena such as, for example, is experienced in the grinding of ores by wet process. Similarly, the present alloys have excellent wear resistance when submitted to hot oxidation phenomena such as, again by way of example, isexperienced during the grinding of slags.

While preferred embodiments of the invention have been described various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

What is claimed is:

1. Heabtreated cast ferrous alloys having a high resistance to wear and consisting essentially in weight percent of:

Percent Carbon 2.3-5.0 Chromium 3310-511) Manganese 0.15-1.5 Silicon 0.151.5 Molybdenum 2.5 Niobium 0-2.5

the balance being iron but containing impurities, such as sulphur and phosphorus up to 0.15%; wherein the total weight percent of carbon and chromium satisfy the relationship Percent Cr-8Xpercent 0:11 to 16;

Percent C 3 Cr 37 MD. -1 0.5

3. The ferrous alloy according to claim 1 further containing an eifective amount of molybdenum up to 2.5%.

4. The ferrous alloy according to claim 1 further containing an effective amount of niobium up to 2.5%.

5. The ferrous alloy according to claim 1 further containing an effective amount of both molybdenum and niobium up to 5%, the content of neither molybdenum nor niobium exceeding 2.5%.

6. The ferrous alloy according to claim 2 further containing an effective amount of molybdenum up to 2.5%.

7. The ferrous alloy according to claim 2 further containing an effective amount of niobium up to 2.5%.

8. The ferrous alloy according to claim 2 further containin g an effective amount of molybdenum and niobium up to 5%, the content of neither molybdenum nor niobium exceeding 2.5%.

References Cited UNITED STATES PATENTS 1,245,552 11/1917 Becket -126 A 2,268,426 12/1941 Schlumpt 75-126 A 2,773,761 12/1956 Fuqua 75-126 A 2,709,132 5/1965 Giles 75-126 A 3,024,104 3/1962 Brennan 75-126 A HYLAND B IZOT, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3834950 Dated S eptember 10, 1974 Inventor(s) Michael Feltz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby correctedas shown below:

FORMAL DRAWINGS; Note that Figures 2-5 on the I second page of drawings should be deleted Signed and sealed this 17th da y of December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents uscoMM-oc 60816-P69 FORM PO-105O (1o-ssi 0.5. GOVIINIIIIT HUNTING OFFICE I'll 0-318-334, 

